1. Technical Field
The present disclosure is directed to a harmonic rejection mixer having an up-conversion mixer and a down-conversion mixer.
2. Description of the Related Art
Electronic mixers are devices used for translating a signal from a first frequency to a second frequency. They are commonly used in transmitters and receivers of communication systems, such as radio transmission systems. On the transmitter side, a base band signal is translated from a base band to a carrier frequency, often denoted as radio frequency fRF. Accordingly, on the receiver side, the incoming signal is translated from the radio frequency to the base band again for being output to the user. These translations commonly have two stages. For example, on the receiving side, the received signal can be translated from the radio frequency to an intermediate frequency fIF first, because some of the signal processing, such as amplifying or filtering, may be done more easily/efficiently at this frequency. From the intermediate frequency, the signal is translated to the base band in a subsequent step. For each of these translation stages, a mixer is used. The mixing operation involves the multiplication of the respective information carrying signal with a local oscillation signal having a local oscillation frequency fLO.
For the translation from the radio frequency fRF to the intermediate frequency fIF, the local oscillation frequency fLO=fRF−fIF or fLO=fRF+fIF is used. An according mixer is illustrated in FIG. 1. A square wave signal is often used as the local oscillation signal. However, such a square wave signal includes not only a spectral component at the local oscillation frequency fLO, but also at odd harmonics of fLO, i.e., at odd multiples of fLO. As no or only low performance band pass filters around the desired radio frequency fRF are commonly used before the mixing operation, the multiplication with the local oscillation signal introduces further spectral components of the received signal, which do not have the desired radio frequency fRF, into the intermediate frequency fIF. This degrades the signal quality at the mixer output. In modern, high performance receivers, such a signal degradation due to undesired components being mixed onto the mixer output frequency is highly undesirable.
Accordingly, it would be beneficial to provide a mixer architecture, that reduces the signal degradation caused by the harmonics of the local oscillation signal.